(LTT) Filler Wires

The lower transformation temperature of filler weld
metal results in considerably lower residual stresses as
shown clearly in Figure 5. Moreover, LTT filler wires
may introduce significant longitudinal compressive
residual stresses within the fusion zone [20]. Similarly,
numerous recent works conducted to investigate the
effect of M S temperature of the filler wire on the
residual stresses developed within the weld region of
high strength steels [1, 8, 15, 17, 18, 20, 21] also
indicate that the amount of distortion developed may be
reduced and even compressive residual stresses can be
induced with the use of these newly developed LTT
filler materials.
It is thus clear that weld metals with intermediate C
contents (i.e. sufficient weldability) has a better chance
to benefit from martensite expansion. Thus, it can be
said that alloyed weld metals (low M S temperature)
with an intermediate carbon content (decrease of the
stresses due to martensite expansion and good
weldability) yield low residual stresses and distortion.
The welded structures with lower residual stresses also
display better properties such as fatigue life [5-7, 9, 10]
and cold cracking [22] with some degradation in
fracture toughness [23]. However, as pointed out
earlier, the fracture toughness of the recently developed
LTT filler alloy has been found satisfactory [12].
Futhermore, several workers [11, 17] have studied the
effect of the martensitic transformation expansion of
weld metals on welding distortion and reported that the
welds produced using LTT filler wires exhibit lower
distortion compared to those experienced using
conventional filler wires.
Figure 3. Microstructures of two-layer welds in S690 Q steel
produced with a LTT (10% Ni-10% Cr) filler: a) macro-section,
b) transition between root and final pass, c) light coloured
segregation of Ni (retained austenite) between martensite in final
pass, and d) martensitic structure in root pass [15]
Figure 4. Variation of residual stress and strain developed during
cooling using conventional and LTT filler wires: a) strain and b)
stress [10]
4. Fatigue properties
High tensile residual stresses forming in high strength
steel welds accelerate the fatigue crack propagation and
reduce fatigue life of the weldment [7]. Another reason
for reduced fatigue strength of weldments is the stress
concentration [24]. The basic approach for improved
fatigue strength in high strength steel welds is to avoid
the joining in highly stressed areas. When this is not
possible then the following measures are usually taken
[7, 25]:
reducing stress concentration by improved
structural design,
improving weld quality (e.g. geometry or
introduction of compressive residual stresses)
by post-weld treatment of critical welds, such
as grinding of weld toe, hammer peening and
shot blasting,
replacing welded connections by mechanical
fastening methods, for instance bolted
connections.
However, all of these methods increase the production
time and cost. Another way of reducing tensile residual
stresses or even introducing compressive residual
stresses is the use of newly developed low
transformation temperature (LTT) filler materials as
earlier mentioned.
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